Arduino with Bluetooth serial port.  MyRobotLab on Android uses this to connect to Arduinos or Arduino clones.  A client .PDE will be needed for the Arduino.  ArduinoSerialBare.PDE can be used.

• Install the ArduinoSerialBare.PDE with the Arduino IDE.
• Install MyRobotLab.apk onto the Android device.
• Use MyRobotLab to start an ArduinoBT Service.
• Connect the ARduinoBT Service to the Arduino

Tutorials
Connecting an Android Phone to Arduino using Bluetooth